One of the fundamental problems in polymer composite technology is that the addition of organic or inorganic particles to a transparent polymeric matrix results in a loss of transparency, or generation of opaqueness, due to an increased degree of scattering resulting from the presence of the particle inclusions. In general, the scattering strength (measured in terms of the scattering cross-section) of homogeneous particles of a given size embedded within homogeneous media increases with increasing difference in refractive index between the suspended particle and the embedding medium. In the following discussion we will focus on inorganic particles dispersed in an organic matrix although the parameters can be applied to other mixed media composite structures including organic/inorganic and organic/organic systems.
The particular properties of nano-sized inorganic materials are of central importance to the design of modern composite materials ranging from polymer composites to cosmetic products where particulate additives are added to a matrix material to improve mechanical, thermal, transport, or optical properties. However, in many instances the improvement of one or more specific performance characteristic is compromised by a loss in transparency that results from the scattering of incident electromagnetic radiation, such as, but not limited to, visible light, UV radiation, and IR radiation, by the embedded particle inclusions. This loss of transparency is a consequence of the significantly different refractive index “n” of most inorganic materials and the organic embedding medium. For applications that capitalize on optical transparency, pronounced scattering due to the presence of particulate inclusions presents severe limitations to the maximum concentration of filler particles, as well as the design possibilities, of the organic-matrix composites.
Bohren and Huffman [Bohren, C. F., Huffman, D. R. Absorption and Scattering of Light by Small Particles, Wiley, New York, 1983] showed that for optically isotropic particles with linear dimensions significantly less than the wavelength of light, the particle scattering cross-section is given by:Csca˜Vp2(Δα)2  (1)with Vp denoting the particle volume and Δα the polarizability difference between the particle and the embedding medium.
The scattering cross-section Csca is a measure of the scattering strength of a particle and is defined in terms of energy conservation: the total energy scattered in all directions is equal to the energy of the incident wave falling on the area Csca.
Since |Δα|=|(∈p−∈m)/(∈p+2∈m)|>>0 (with ∈i=ni2 denoting the dielectric constant of medium ‘i’; ‘p’ and ‘m’ represent the particle and embedding medium, respectively) for most inorganic/organic material combinations, significant scattering can arise even for small particle sizes.
The embodiments of the present disclosure allow the physical properties of a composite material to be modified by the added core-shell particles without sacrificing transparency through modification of particle surfaces with polymers to generate core-shell particle architectures with an effective refractive index and/or effective dielectric constant that is substantially equal to the respective refractive index or dielectric constant of the envisioned or targeted embedding medium.